The nature of acquired resistance of treponemal infection has not been elucidated. We have shown that serum or cells obtained from inbred hamsters immune to Treponema pertenue or T. pallidum strain Bosnia A can confer protection on recipient hamsters to homologous treponemal infection. The objectives of the proposed research are to determine: (1) whether specific resistance to treponemal infection can be conferred on recipient hamsters with enriched populations of thymus-derived cells (T cells) or lymphocytes exposed to a well characterized anti-hamster thymocyte serum; (2) the relative contribution of various immunoglobulin fractions of treponemal immune serum (IgG1, IgM, or IgG1 and IgG2) on the response of the hamster to treponemal infection; (3) the temporal relationship between the development and regression of frambesial or syphilitic lesions and the infiltration of T or bursa equivalent (B) cells into these lesions; (4) the in vitro response of lymphocytes obtained at various times after infection of hamsters with treponemes to stimulation by T or B cell mitogens and specific treponemal antigens in the presence of autologous or homologous serum; and finally (5) whether cross-immunity develops among the causative treponemal agents of endemic syphilis, frambesia (yaws) and venereal syphilis. Investigation of the immune response mechanisms by which the hamsters respond to treponemal infection may help to delineate the mechanism by which humans respond to treponemal infection.